Workovers in existing wells can require removal of packers or plugs by milling them out. Other occasions can also occur where there is a need to mill out a tool or even a casing section. If the well is not too deviated, rigid tubing has been used to support a mill and the rotation force provided from surface equipment. Alternatively, where the deviated nature of the wellbore precludes rotation from the surface, the bottom hole assembly includes a mud motor to turn the mill. The bottom hole assembly is still delivered on rigid tubing but such tubing above the mud motor remains stationary, with the output of the mud motor driving the mill below. In either alternative a workover rig must be erected over the well to handle the rigid tubing string for trips into and out of the well. There is a fair amount of expense associated with erecting the rig on site and handling the tubing to assembly and disassemble the string for trips into the well. It would be advantageous if a coiled tubing unit could be used at the surface instead of a workover rig. Being able to use coiled tubing would save time and money for the operator over using rigid tubing. However, the use of coiled tubing creates other issues that are not of concern when using rigid tubing. The main problem is that coiled tubing is considerably weaker than rigid tubing. During milling a reaction torque is created that is passed to the supporting tubing. In the past, milling on coiled tubing has been attempted in small casings that are less than 4-½ inches with equally small mud motors driving the mill. These attempts worked, after a fashion, because the torque output from the motor and the resultant torque reaction from milling was sufficiently small so as to not twist the coiled tubing. If the torque reaction turns the coiled tubing it can raise the mill off the packer being milled or bounce it, resulting in erratic milling. Worse still, the coiled tubing can fail from being over-torqued. For this reason milling with coiled tubing was limited in the past to very small applications, generally with casing sizes fewer than four inches.
The milling process generates debris in the wellbore. Even if a milling job in a larger casing were attempted with small coiled tubing and an equally low powered mud motor, the return flow in the larger casing sizes would reduce the velocity of the returning fluid so as to allow the debris to drop out rather than be carried to the surface for separation with surface equipment. While debris catchers of various designs are known they have operational shortcomings. Some require a separate trip. They generally let the debris-laden fluid passes through an open port on the trip downhole. When the tool is brought uphole, the bypass port is closed and the fluid passes through a screen leaving the debris inside the screen. Some examples of such tools are the H-3015 and the 10084-1 offered by Baker Oil Tools. Some debris catchers can be run in the same trip as the milling equipment but due to the way such tools operate they can't have a mud motor below them. These tools use a venturi effect to direct the cuttings into the tool and generally must be coupled with specially designed mills that create the type of cuttings that will enter this type of debris catcher. One such tool offered by Baker Oil Tool s is the VACS tool. U.S. Pat. No. 6,176,311 illustrated the concepts of central circulation, annulus diversion of debris into the tool, an interior capture area and screen. This design has been improved in the present invention to minimize issues of plugging and damage to the annulus diverter device when running in or removing the tool. Other debris removal tools are described in U.S. Pat. Nos. 5,176,208; 5,402,850; and 6,276,452.
Anchors for tubing downhole are known, as illustrated in U.S. Pat. No. 6,276,452.
The present invention permits small coiled tubing to support large mud motors for big milling jobs. The coiled tubing is anchored in position and the mud motor operates the mill in conjunction with a thruster to keep the mill on the tool being milled. Other variations are envisioned that secure the coiled tubing against torque reaction while allowing the mill to progress and mill out downhole. An improved debris catcher is incorporated into the assembly with greater debris retention capacity and other operational enhancements to improve its operation. These and other aspects of the invention will be more readily apparent to those skilled in the art from the description of the preferred embodiment and the claims that appear below.